Devices are used for continuous hot-dip galvanizing of steel strip, which consist among other things of a continuous furnace and a zinc bath (melt bath). The steel strip is continuously annealed in the continuous furnace. The desired mechanical properties of the basic material are adjusted in this process by recrystallization of the steel. Furthermore, iron oxides formed in a preheating zone are reduced. In a cooling zone coming after the continuous furnace, the strip is cooled down under protective gas to a temperature near the melt bath temperature. The protective gas is supposed to prevent the annealed strip from being oxidized prior to the galvanization, which would significantly impair the adherence of the zinc layer. A so-called furnace trunk is used as a connecting piece between the continuous furnace and the zinc bath.
To prevent this, a device is known for example from JP 7157853 (A) for the removal of zinc vapor in a trunk of a continuous strip galvanizing layout. In order to remove the zinc vapor arising at the surface of the zinc bath, the furnace trunk is provided on both sides of the strip in each case with a single blow-in opening (circulation opening) and, vertically underneath it, a single suction opening on both sides of the strip. In one exemplary embodiment, the suction openings are each designed as a longitudinal slot in a tube, which passes through a side wall of the trunk and extends over the entire width of the steel strip at the top and bottom side of the steel strip. However, on account of the configuration and arrangement of the blow-in openings and suction openings it is to be assumed that this known device cannot adequately prevent the dispersion of zinc vapor in the furnace trunk and as a result a dispersion of the zinc vapor in the furnace trunk is favored.
This has been attributed to the fact that the steel strip in the trunk moving in the direction of the zinc bath may sometimes entrain protective gas downward in an uncontrolled manner, wherein the entrained protective gas takes up zinc vapor at the surface of the zinc bath, which during the rising of the entrained protective gas is condensed or resublimated at the colder inner walls of the trunk and is deposited there as dust.
To prevent this, it is proposed according to WO 2014/006183 A1 to avoid the entrainment of the protective gas. For this, several blow-in openings and suction openings are provided and the distance between the respective blow-in opening and an associated suction opening is chosen such, and the rate of flow of the protective gas emerging from the respective blow-in opening is controlled such, that an entrainment of protective gas in the direction of the zinc bath occurring during the movement of the metal strip is prevented. This is substantially achieved in that a mixed region with both blow-in openings and suction openings is provided. In other words, the region with blow-in openings and the region with suction openings overlap entirely or are intermeshed like a comb.
However, it has been found that the extraction of zinc vapor is sometimes not adequately achieved with this solution. In particular, it has been found that the blocking of the rising zinc vapor still needs further improvement, due to too direct a mixing of zinc vapor and protective gas in the solutions of the prior art. It has also been determined that an inhomogeneous temperature distribution may still prevail partly in the trunk, which favors a depositing of metal vapor.
Thus a need exists for devices and methods of this kind with which the extraction of metal vapor by means of the protective gas can be improved and the dispersion of metal vapor can be reduced.